1. Field of the Invention
The present invention relates to an automotive anti-skid control system which prevents skidding and thus provides maximum effective braking and assures steering effect on turns by optimally controlling each wheel-brake cylinder pressure, and particularly to a system which can optimally control a braking force applied to each of rear wheels during braking, depending upon various running conditions of the vehicle without providing any expensive acceleration sensors.
2. Description of the Prior Art
As is generally known, on conventional automotive anti-skid control systems which can prevent wheel-lock during braking, each wheel-brake cylinder pressure is optimally controlled by adjusting a slip ratio at the slipping less-traction wheel towards a predetermined reference slip ratio. The slip ratio at the slipping less-traction wheel which is subjected to anti-skid control, is generally derived from the ratio of the deviation between a vehicle speed and a wheel speed detected at the slipping less-traction wheel with respect to the vehicle speed. On the other hand, the predetermined reference slip ratio means a desired ideal slip ratio advantageous to provide a maximum traction (maximum possible friction between the road surface and the tire) and to assure both a steering effect on turns and a shorter braking distance during braking. For instance, when the derived slip ratio at the slipping less-traction wheel exceeds the reference slip ratio, the brake-fluid pressure to the associated wheel-brake cylinder is reduced so as to prevent skidding or wheel-lock. In contrast, when the slip ratio becomes less than the reference slip ratio owing to the above-mentioned reduction of the wheel-brake cylinder pressure, the wheel-cylinder pressure is built up again so as to adjust the slip ratio towards the reference slip ratio. The braking force at the slipping less-traction wheel, which is subjected to anti-skid brake control, can be adjusted by automatically controlling the braking operation as if automatic pumping brake action is executed in cycles.
In case that the anti-skid control is made with respect to left and right rear wheels (unsteered wheels) during braking when the vehicle is traveling on a so-called splits road in which friction coefficients of left and right road surfaces are remarkably different from each other, for example the right-hand side road surface is a low-.mu. road, whereas the left-hand side road surface is a high-.mu. road, there is a braking-force difference between rear-left and rear-right wheels owing to the split-.mu. road. During braking on the split-.mu. road, the wheel speed of the slipping less-traction rear wheel (of a high possibility of wheel-lock and of a greater slip ratio) tends to become slower, while the wheel speed of the almost non-slipping greater-traction rear wheel of a less slip ratio tends to become faster. In this case, the conventional anti-skid control system controls rear-left and rear-right wheels simultaneously and in common with each other at the same anti-skid control mode in which the respective wheel-brake cylinder pressures of the rear wheels are controlled on the basis of data indicative of a slip ratio detected or estimated at one of rear wheels, namely the slipping less-traction rear wheel, (i.e., the slower-rotating rear wheel). Such an anti-skid control common to the rear wheels is often called a "select-LOW method" or a "select-LOW process". The select-LOW method is effective to enhance a steering stability and a controllability by reducing undesired yawing moment, when the anti-skid control is executed simultaneously at the rear wheels in common with each other during braking on the split-.mu. road. However, in the event that the rear wheels are simultaneously controlled in common with each other by way of the select-LOW method during braking on turns, the faster-turning outer rear wheel is also controlled in accordance with the same anti-skid control mode as the slower-turning inner rear wheel, irrespective of the fact that the outer rear wheel has a less possibility of wheel-lock than the inner rear wheel owing to shift of wheel-load from the inner rear wheel to the outer rear wheel. In this case, the braking force to be produced at the outer rear wheel tends to be suppressed unintendedly and excessively, thereby reducing the total braking force of the vehicle (the four wheels) and thus increasing the braking distance. To avoid this, Japanese Patent Provisional Publication (Tokkai Heisei) No. 4-339065 (corresponding to Japanese Patent Application No. 3-139436, and to U.S. patent application Ser. No. 883,017, filed May 14, 1992 and assigned to the assignee of the present invention) has disclosed an anti-skid control depending on the magnitude of lateral acceleration. In the U.S. patent application Ser. No. 883,017, the system employs a lateral-acceleration sensor for detecting a lateral acceleration exerted on the vehicle. When the magnitude of the lateral acceleration detected is small, rear-left and rear-right wheels are simultaneously controlled in accordance with the common anti-skid control mode based on a comparatively greater slip ratio detected at the slipping less-traction rear wheel. In contrast, when the magnitude of the lateral acceleration detected is great, the rear-left wheel and the rear-right wheel are controlled independently of each other on the basis of the respective slip ratios. In other words, the prior art system disclosed in the U.S. patent application Ser. No. 883,017 teaches the provision of a lateral-acceleration dependent anti-skid control mode selection means. In general, such a lateral-acceleration sensor is expensive. In order to enhance reliability in the anti-skid control, the prior art system may also require a fail-safe system in consideration of failure in the lateral-acceleration sensor, thus increasing total production costs of anti-skid control systems. To avoid this problem (the use of an expensive acceleration sensor), Japanese Patent Provisional Publication (Tokkai Heisei) No. 8-188138 (corresponding to Japanese Patent Application No. 7-4468, and to U.S. patent application Ser. No. 583,884, filed Jan. 11, 1996 and assigned to the assignee of the present invention) has disclosed an automobile anti-skid control system which can optimally control a braking force applied to a turning outer rear wheel during braking when the vehicle is rounding a curve and effectively suppress undesired yawing moment exerted on the vehicle during traveling on a so-called split-.mu. road, without employing any lateral-acceleration sensors. The U.S. patent application Ser. No. 583,884 teaches an anti-skid control that an actuator associated with a controlled rear wheel, which is subjected to a braking-force control, is controlled on the basis of the lower value of a wheel-speed indicative signal value of the controlled rear wheel and a wheel-speed indicative signal value of the diagonal front wheel located on the vehicle diagonally to the controlled rear wheel, during the braking-force control for the controlled rear wheel, in order to eliminate lack of the braking force applied to the rear outer road wheel turning during braking on turns without providing a lateral-acceleration sensor. That is to say, the anti-skid control system disclosed in the U.S. patent application Ser. No. 583,884 can execute a so-called select-LOW process between one diagonal road wheel pair (front-left and rear-right road wheels), or between another diagonal road wheel pair (front-right and rear-left road wheels). The rear wheel-brake actuator control based on the select-LOW process between a diagonal road wheel pair is very advantageous during braking on turns, since the braking force applied to the faster-rotating outer rear road wheel (of the second lightest wheel load) can be maintained at a value essentially equivalent to the braking force applied to the slower-rotating inner front road wheel (of the second heaviest wheel load) and thus a braking distance can be effectively decreased. The rear wheel-brake actuator control based on the select-LOW process between a diagonal road wheel pair is advantageous during traveling on the split-.mu. road, since the braking force applied to the rear road wheel rotating on the highs road can be properly adjusted toward a comparatively small braking force as applied to the front road wheel rotating on the low-i road and thus the braking-force difference between the rear-left and rear-right wheels can be effectively suppressed. However, in case of the anti-skid control system disclosed in the U.S. patent application Ser. No. 583,884, there is a possibility of the braking-force difference between rear-left and rear-right road wheels owing to the previously-noted rear wheel-brake actuator control based on the select-LOW process between a diagonal road wheel pair, during braking in a straight-ahead high-speed driving state of the vehicle. Additionally, when the rear wheel side is extremely heavily loaded with heavy luggage in comparison with the front wheel side, there is a tendency of lack of a braking force applied to the controlled rear wheel during the braking-force control for the rear-wheel side, owing to the previously-noted rear wheel-brake actuator control based on the select-LOW process between a diagonal road wheel pair.